Understanding fundamental biology of settlement and growth for fouling organisms and finding ways to control or minimize fouling have been goals of much research and development. The subject is complex, ranging over areas that include fundamental biology, polymer science, coatings technology and, ultimately, paint systems.
Two major components of marine fouling are “soft” organisms such as seaweed and “hard” species including barnacles, mussels, oysters and tubeworms. Depending on factors such as temperature, salinity, location and time of year, fouling can rapidly roughen a ship hull, greatly increase drag, and cause increased fuel consumption to maintain a given speed. Hard fouling can take the form of invasive species such as zebra mussels, which are also notorious for fouling power plant heat exchangers, and are opportunistic organisms in aquaculture.
Various biocides have been incorporated in coatings so that by slow release fouling organisms are killed in the settlement stage. However, ships spend time in ports that are estuarine environments resulting in leached biocides affecting non-target species. For example, 20 ng/1 tributyltin species was found to cause defective shell growth in oysters.
Such developments have led to research and development on environmentally benign methods to control fouling.
While the perfect nonfouling surface remains elusive, a practical target is a coating from which soft and hard fouling can be easily removed. Such coatings that minimize adhesion without release of toxicants or bioaccumulative species are referred to as foul release coatings (FRCs) or abhesive coatings. Applications for such abhesive coatings include not only control of marine biofouling but, because of their benign nature, include a broad scope applications such as in medical devices and in biotechnology to limit bioreactor fouling. Current compositions do not adequately address the problems.
Ice accumulation is a serious problem for aircraft, ship superstructures, wind turbine blades, power transmission lines and similar exposed structures. Icing has led to deadly accidents as well as material loss, reduced performance or interference with normal operations.
It is well known that airfoil icing disrupts airflow, reduces lift, and jeopardizes control. Ice accumulation must be removed before takeoff, typically with ethylene or propylene glycol-based fluids or foams. A gel may be used to prevent fluid runoff and retain the de-icing agent. Based on a 2009 EPA document, more than 25 million gallons of de-icing agents were applied at commercial US airports each year. De-icing agents are normally not recycled and are discharged directly into waste water systems. Such discharges have caused increased biological oxygen demand (BOD) and total organic content (TOC) in groundwater. Though not common, under certain atmospheric conditions, ice accumulation may also occur in flight. Depending on airplane design, de-icing may be accomplished by routing hot air from engines. This method obviously increases fuel cost. Icing of helicopter rotor blades has similar effects and consequences. Deicing rapidly rotating helicopter blades is not known to have a practical solution.
For “wind farms”, the wind turbine blade profile is critical for optimum power production and durability. Even slight icing alters the aerodynamic blade profile and diminishes power production. Additional ice accumulation can drastically change the blade profile and increase the structural loading on the rotor and tower leading to catastrophic failure.
Power companies often suffer billion dollar losses in major winter storms. An ice storm in Canada in 1998 caused the destruction of 130 transmission towers and 30,000 utility poles and resulted in 3 million customers being without electricity (a quarter of them were still without power after three weeks). It cost power companies C$ 1 billion to make repairs caused by this damage. Telecommunication companies face similar challenges for above ground cables in winter months.
Currently used active methods for de-icing include de-icing fluids for aircraft discussed above and resistive heating where ample power is available such as wind turbines, automobile windshields, and refrigeration units. Resistive heating is costly to implement and reduces net power generated from wind farms. Passive de-icing methods such as icephobic and ice-release coatings are based on silicones or fluoropolymers. Silicones are known for their weak mechanical properties and high cost. Fluorocarbon polymers, if used in the neat form, are even more expensive than silicone materials.
It is logical to think that ice cannot form if water does not wet the surface. Therefore, superhydrophobic surfaces have been investigated to achieve icephobic surfaces. In most cases, such surfaces require careful microstructural fabrication or electrospinning to generate specific complex microstructures for samples that have dimensions of a few square centimeters. Such complex processes are not applicable for large surface areas, at least at present.
A common but mistakenly held notion is that polytetrafluoroethylene (PTFE) or “Teflon” should be good for ice release. Teflon and similar semicrystalline fluoropolymers are processable at high temperatures to generate “non-stick” surfaces for cookware and the like. However, such high temperature processes are not applicable for large area coating technologies. Secondly, polymers made of long fluorocarbon chains (>C6) are degradable to perfluorooctanoic acid (PFOA) that persists indefinitely in the environment. PFOA is bioaccumulative and is a proven carcinogen. Again, current technologies are inadequate.